Give yourself a pat on the back because you deserve it, smart girl.  Secondly, it's great that you've got so much goin' on because it's a good way to gain friends and recognition, so props for that, too. But there's one huge problem here. You're forgetting about taking care of someone really important...Y-O-U!

I'm sure that your mom and teachers are proud of how smart you are and just want your success, but they also gotta understand that you have limits like every other human being.  No matter how smart you are, you are only one person and I'm sure that both your mom and your teacher will understand that. You cannot do everything and that is OK! 

So how do you solve this sticky sitch? You're gonna have to express yourself in order for anyone to understand you. If you don't speak up, you might take on too much and go on overload. You'll either become less effective and the quality of your work will start to fall or you'll wear yourself out completely. Eek!
Go to your mom ASAP. Tell her that you love how much she believes in you and that you value her support, but you absolutely cannot join another group because you cannot find the time to do it. Don't be afraid to tell your teacher this, too. Next, pick and choose activities by the order that they're most important to you. Do you totally love French club but could ya live without debate team? It's ok to quit something that your heart's not into. Think about it this way – would you rather run for pres of your fave club or be in two clubs, wearin' yourself out? It's all about priorities!

And, aside from schoolwork, you have friends, family and just good ol' fun stuff to make time for, too.  So, lighten your load and start feeling happy about life!
 
Hugs,